Monitoring a rotational angle sensor

ABSTRACT

The invention relates to a monitoring method in which two variables of angle-dependent, amplitude-modulated carrier frequency signals are emitted on orthogonal windings ( 12   a,    12   b ) of the rotational angle sensor and an evaluation is carried out by an input amplifier (V 2, 11 ) which emits an output signal that is essentially zero when the signals of the variables are generated according to functional usage. Alternately, an interference signal is generated at one-second intervals in both windings ( 12   a,    12   b ) of the rotational angle sensor, the result of which is being evaluated and monitored by means of an input amplifier (V 2, 11 ). An interference display signal is generated at the output of the input amplifier (V 2, 11 ), said interference display signal being not equal to zero if no error or interruption is present in the system and equal to zero if an error or interruption is present in the system.

CROSS-REFERENCE TO RELATED APPLICATIONS

The application is a U.S. National Stage Application of InternationalApplication of PCT/EP2009/056335 filed May 26, 2009.

FIELD OF THE DISCLOSURE

The present invention relates to a method and a device for monitoring arotational angle sensor on an electric machine and is used, forinstance, in measurement systems using a resolver.

BACKGROUND

For position detection of moving machine parts and drives resolvers haveproven to be reliable and cost-efficient in industrial applications. Ifthe controlling of the machine or the application requires adetermination of an angle very frequently resolvers are used due totheir robust construction. The resolver is an absolute position or anglemeasurement system that is based on an inductive principle and is in itsconfiguration similar to an electric motor having precision windings.While the stator of the resolver bears two winding groups whose windingplanes are perpendicular and are spatially offset to each other(W_(sin), W_(cos)), the rotor comprises a rotary transformer that issupplied at its primary side by an excitation winding (W_(Ref)) and thatis inductively coupled with its secondary side with the stator windings.

According to the principle of operation a resolver is configured suchthat at the input side an excitation with a carrier frequency occurs andsuch that due to the resolver motion this signal is amplitude-modulatedand is transformed to the output side. Due to the orthogonal arrangementof the windings the amplitude-modulated output signal have a phase shiftof 90° to each other and are available for further analysis. There areseveral analysis methods for resolvers.

The application of a resolver may be diverse. Sometimes there ismeasured and integrated the angular speed and not the rotation angle. Itis, however, essential, that one is able to determine, either directlyor indirectly, for example by integration, the angular position of therotor with respect to the stator. The angular information or therotation speed information is supplied to a frequency inverter or to anyother electronic motor controller and is used therein for controllingthe rotation speed and/or the torque.

When the resolver fails for some reason the electronic motor controllerdoes not receive a signal. A failure is to be understood as any state inwhich the correct rotation angle signal is not received by the motorcontroller. This can be, for instance, an interruption of the signalline between the resolver and the motor controller. In case that themotor controller does not receive the correct signal, this will resultin a wrong behaviour during operation. For example, it may be that amotor controller upon using a current vector control regime withrotation speed feed back as is usual nowadays adjusts the rotation speedof the motor to its maximum, when the signal line is interrupted. It isevident that such operating states are not desirable.

In the prior art several devices and methods are known in order toidentify such failure situations in the context of rotational anglesensors so as to subsequently take measures for error correction or atleast for error treatment. For example the rotational angle sensor maybe configured in a redundant manner, that is, a second rotational anglesensor may be provided and the results of both rotational angle sensorsmay be compared with each other. Moreover, the operation of therotational angle sensor may be monitored by means of a built-in controlelectronic.

Furthermore, it is possible to detect a wire breakage in the conductorsto and from the resolver that is used in electric machines for angledetermination.

In order to identify wire breakage-based failures during thedetermination of angles typically the input and output signals of theresolver W_(sin), W_(cos), W_(Ref) are electrically fed back and thecurrent flow in these windings is analysed. In this manner the conductorlines to and from the resolver including the windings connected theretoare monitored. A direct monitoring of the windings of the rotarytransformer in the rotor is, however, not possible in this manner. Amonitoring of the winding can only be accomplished indirectly and inthis case the used resolver analysis procedure has an essentialinfluence on the applicable options.

A usual resolver analysis method is the so-called backwards procedure inwhich the orthogonal windings of the resolver as W_(sin) and W_(cos) aresupplied with current such that no voltage is induced in the excitationwinding as W_(Ref). If the resolver rotor is rotated in this state ofsignal equilibrium, then this equilibrium state is disturbed and avoltage is transferred into the winding W_(Ref). In this procedure thisvoltage is supplied to a controller (the so-called tracking controller),which corrects the voltage amplitudes at the orthogonal coils such thatthe voltage across the excitation winding again becomes zero Volt andthe signal equilibrium is re-established. Since the rotor of theresolver is a rotary transformer having respective windings, naturallyonly AC signals can be transmitted by the transformer. For this reasonthe input signals of the resolver are amplitude-modulated carrierfrequency signals.

The complete monitoring of a system that is based on the above describedtracking control principle is difficult. In such a system in the “normalstate”, i.e., when the tracking control loop is adjusted, about zeroVolt are measured at the input amplifier. The same voltage amplitude,however, is also present when a wire breakage occurs in the windings ofthe rotary transformer of the resolver. Therefore, errors within theresolver are not detectable.

One possible approach for detecting such an error is the usage of aso-called test mismatch angle. In this manner the tracking control loopdoes not receive the target value being equal to zero Volt, but beingnot equal to zero. In such a system the signal at the input amplifier isalso unequal to zero in the adjusted state and a wire breakage wouldthus be detectable.

Typically, the input amplifier is, however, dimensioned such that evenlow mismatch angles that already occur during, for instance, dynamicevents, will saturate the input amplifier. This high amplification isnecessary, since only with this measure the control loop is able tofollow highly dynamic motions

Measurements revealed that the test mismatch angle has to be in theorder of magnitude of about 1° in order to be unambiguously detectable.Unfortunately, this fact and the required high amplification of theinput amplifier cause an evidently asymmetric modulation of the inputamplifier, thereby negatively affecting the control loop stability.

This known technique may thus not always be applied.

SUMMARY OF THE DISCLOSURE

It is an object to provide a monitoring of a rotational angle sensor inan electric machine, and mitigate adverse influence on the control loopstability and security.

This object is solved by a monitoring method of a rotational anglesensor and by an apparatus for monitoring a rotational angle sensor.

The invention is able to detect an error and/or a wire breakage in thewindings of the rotary transformer of the rotational angle sensor, e.g.,a resolver.

According to the disclosure the monitoring of a rotational angle sensoris accomplished by generating an interference signal in one-secondcycles or intervals. This well-directed interference signal is providedin an alternating manner for both windings of the rotational anglesensor the SIN and the COS winding.

After evaluation via an input amplifier and the monitoring aninterference indication signal at the output of the input amplifier isconverted into a signal that is not equal to zero Volts, when no erroror no breakage occur in the system, and is converted into a signal beingequal to zero Volt, when an error or a breakage in the system exists.

After correcting the error a further evaluation and monitoring areperformed.

For the monitoring of the rotational angle sensor in the electricmachine a so-called failure detection block is important that consistsof an interference signal generator, a signal analysis block and aswitch. It is connected to the carrier frequency generator.

During the monitoring the interference signal generator generates inone-second intervals, in an alternating manner for both windings of therotational angle sensor, a specific interference signal analysed via aninput amplifier and being monitored. When the interference indicationsignal at the output of the input amplifier is a signal that is notequal to zero, there is no failure or breakage in the system. The signalcorresponds to zero Volt, when an error or a breakage exist in thesystem.

The circuit of the monitoring system and the method of monitoringfunction in this way. They may be combined.

For explaining a rotational angle sensor that in one embodiment isconfigured as a resolver it should be mentioned that it represents amagnetically coupled system, as is expressed by the previously describedsituation of a rotary transformer. There is a rotating winding and thereare two stationary windings, the latter one is called stator winding.The rotating winding is called rotor winding or also “excitationwinding”. For the example of a resolver as a rotational angle sensor, inshort also referred to as “encoder”, the following explanation should begiven.

The resolver comprises an excitation winding at the primary side (as arotating winding) and two secondary windings of the stator, orthogonallyarranged to each other (as stationary windings) and mechanicallyemulates a trigonometric addition theorem.

In the “backwards procedure” the resolver is not supplied with current,as actually structurally intended, via the excitation winding, but thetwo secondary windings are supplied with current and the transfer resultis measured in the excitation winding via the input amplifier. This isthe result of the response to the carrier signal.

The current injection of the two secondary windings is done such thatthe fields of the two secondary windings neutralize each other and novoltage is induced in the excitation winding. If, however, some voltageis induced in the excitation winding a control loop corrects theamplitudes of the two (stationary) secondary windings, until the signalat the excitation winding is again equal to zero.

The mathematical basis is the addition theorem that is emulated by theresolver.

If it is referred to the signal zero, this signal is to be understood asan alternating (AC) signal that the carrier frequency signals inducefrom the stator windings in the excitation winding (as a rotary winding,here used as a receiver winding) as an alternating signal. The controlloop provides for the steady state and also effects that no alternatingsignal exists at the input of the input amplifier, or also at the outputthereof, i.e., in this sense the alternating signal is zero (ameasurement via the RMS value is recommendable).

This zero condition is also obtained, as previously described in theprior art, when the transfer ratio ü is faulty and an error or a wirebreakage exists in the winding system of the resolver. In this case theclaimed invention ensures that via short pulses, referred to as“generating interference signals in cycles of a second”, asymmetricconditions are injected that produce an alternating signal. As aconsequence of the interference signal the indication signal at theinput and also at the output of the input amplifier will be generated.It is an interplay being not equal to zero in the sense that analternating signal with an amplitude and an effective value is present,and it may be assumed that no failure or wire breakage exists in thesystem of the resolver. If such a failure would exist, an alternatingsignal would not be generated at one or another interference signal andthe signal would be equal to zero, in other words, it would not exist asan alternating signal. In this case it can be monitored that a failureor a wire breakage is present in the system of the resolver.

In order to be symmetric the interference signal is alternatelygenerated in one winding and in the other winding. In such a mannererrors in the two windings can be detected. The wording that the signaloccurs in one-second intervals or cycles means that is has a post-signaltime period that is significantly greater than the active signal timeperiod in which the interference signal actively interferes with avoltage of the windings in the meaning that it is suppressed or thecarrier frequency is switched off or suppressed. This happens in the wayof a Dirac pulse that is symbolically represented for the interferencesignals by δ, wherein i=1, . . . , n symbolizes a sequence ofinterference signals whose pulse width is small compared to therepetition time. This is the meaning of the interference signal inone-second intervals that affects alternately in one or the other of thetwo windings. The signal voltages for these windings, described later onin the specification as U₁ and U₂, are set alternately to zero and inthis way the transfer of one of these signals to the excitation side isinduced if, and as long as, no failure in the sense of a wire breakageor an error is present. If on the other hand a failure is present thenthe transformation ratio of the windings (stator winding to rotorwinding and vice versa) is equal to zero and the voltage at the inputamplifier, which connects to the excitation winding, also becomes zero.The control loop would loose its effect, i.e., it would be provided witha measurement signal that indicates an adjusted state although atechnical error exists in the resolver.

The switching off of the carrier frequency of the generator, however,would disturb the controller. During this time interval the controlleris halted, which is accomplished by a switch. It opens the feed back,temporarily terminates the control function. The supply of a null signalas a control error representing a non-received signal voltage at therotating winding indicates to the control loop that an adjusted stateexists, since the target value of the control loop is configured as atracking controller and it receives as a target value the value zero.During this “immobilization” or during the halt of the controller, whereno measurements are performed, the interference signal tests, byswitching off one of the two alternating signals at a respective one ofthe two stator windings, whether the transformation ratio of thewindings of the rotary transformer (in the meaning of the resolver) isstill operational and present, which is represented at the output of theinput amplifier by providing a voltage that is detected by a circuitthat is to be described as an interference detection circuit or signalanalysis circuit. It is supplied with the interference signal so that itknows when it can detect an error, and it is also provided with theoutput signal of the input amplifier so as to enable to determinewhether a desirably existing signal at the input amplifier is received.Also during this time interval of the interference state, i.e., duringthe presence of the interference signal, the control is, as discussedbefore, stalled but only to such an extent that it does not perform oradjust any transient oscillations.

Transmitting two actuating variables to the stator winding of therotational angle sensor represents angle dependent amplitude-modulatedfrequency signals that are provided by an amplitude modulator. Thetransmitting is to be understood such that these actuating variables areprovided by the amplitude modulator as a sine variable and a cosinevariable for a sine winding and a cosine winding, respectively, of therotational angle sensor.

The thus generated signal at the rotating winding, when operated inforward direction, referred to as excitation winding, when operated inbackward direction, referred to as receiver winding, forms a signalreaction that is adjusted by the control loop during normal operation,that is, when the resolver is operable, such that the output signal atthe excitation winding changes to zero. Also in this case an alternatingsignal is meant, which alternating signal is not present and has noeffective value. The analysis in the input amplifier functionallyensures for the tracking controller and the present control loop thatthe error variable becomes equal to zero and the output signal at theinput amplifier positioned downstream of the receiver winding isadjusted to zero.

If the interference signal is generated, one cannot speak of a properfunctional generation of an output signal at the “excitation winding”used as receiver winding for a short time during the duration of theinterference. Nevertheless, there is also an analysis that occurs viathe same input amplifier that provides an output signal of zero Volt,when an error or a breakage exist in the system. It would not be equalto zero when the signals for the manipulated variables are generated ina functional manner and the interference signal does not activelyintervene.

An interaction of the control loop is expressed by the controlling to avalue zero at the excitation winding, according to the invention used asa receiver winding, which is referred to as normal operation.

The backward procedure or the usage of a resolver in a backward mode isexpressed in such a manner that two actuating variables are “sent” tothe stator windings (supplied thereto) and the signal responses areanalysed at the excitation winding used as receiver winding, to whichend the input amplifier is used. This functional state including thetracking control loop uses a difference unit, the controller itself, anintegrator and said amplitude modulator. It comprises two signal blocks,one for cosine and one for sine, and two multipliers for modulation withthe carrier signal. This carrier signal is output by a frequencygenerator that is denoted as “carrier frequency generator”. Via twoseparated amplifiers the stator windings of the resolver are processed,which are arranged in the rotational angle sensor as two windingsorthogonal to each other.

The induced signal in the excitation winding used as receiver winding issupplied to the input amplifier that outputs, for the adjusted state, anull output signal as “no alternating signal” and provides this to ademodulator that forms in the feed back the closed loop for thedifference unit. Here it is provided functionally acting switch thathalts the control loop. The halting is accomplished by providing anadjusted equilibrium state for the difference unit. The halting thusensures that the control loop is not affected by the injectedinterference variable. During this time a failure detection blockprovides for a detection of the signal at the output of the inputamplifier. During the defined time intervals, i.e., during the durationof an interference signal, a signal in the form of an alternating signalthat is not equal to zero should be applied here. In a sense theinterference signal is included in the input signal of the inputamplifier, however in an opposite imagined direction.

The switching off of one of the actuating variables signals or of theircarrier frequency ensures that associated therewith an alternatingsignal is present at the input amplifier, when the resolver istechnically functioning properly and no error or breakage exists in thesystem.

The failure detection block has a signal analysis block that analysessaid input signal of the input amplifier or its output signal afterpassing through the demodulator and while the control loop isdeactivated. This temporary deactivation corresponds to the generationof the interference signal in one-second intervals. If the failuredetection block detects the non-presence of an output voltage at theinput amplifier during the active phase of the interference signal, thefailure detection block outputs an error signal or failure indication F.

The switch is disposed in the feed back branch of the controller. Itblocks or switches off the measured signal. Preferably, the switch mayprovide a null signal to the difference unit of the control loop whilethe interference signal is effective.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following the invention will be discussed in more detail byreferring to embodiments in the drawings circuit examples and byreferring to methods of operation of these examples.

FIG. 1 is as schematic view of a monitoring device including a resolverR.

DETAILED DESCRIPTION OF EMBODIMENTS

The device for monitoring a rotational angle sensor at an electricmachine has a control difference unit 1, a controller 2 and anintegrator 3 that is connected with the amplitude modulator 4.

The amplitude modulator 4 consists of a sine signal block 5 and a cosinesignal block 6, as well as multipliers 7 a and 7 b, and a carrierfrequency generator 8. The latter multipliers are connected to the(stationary) windings W_(sin), W_(cos) (stator windings 12 as 12 a and12 b) of the rotational angle sensor R via two amplifiers 10 a, 10 b tosupply actuating values to the windings.

The signal of the excitation winding W_(Ref) as a rotating winding 13 issupplied to a demodulator 9 via the input amplifier V2 (or 11).

There is provided a failure detection block 14 that consists of theinterference signal generator 15, a signal analysis block 16 and aswitch device as a switch 17 and that is connected to the carrierfrequency generator 8 for transmitting signals δ_(i), i=1, 2, . . . n.Likewise, the interference signal generator 15 is coupled to the switchdevice 17. Also, δ_(i) is coupled with the signal analysis block 16.

The embodiment of FIG. 1 illustrates a monitoring method in the backwardprocedure (backward mode), wherein the rotational angle sensor isconfigured as a resolver and the orthogonal windings 12 a, 12 b of theresolver stator are supplied with current such that no signal istransmitted to the excitation winding W_(Ref)—used as receiverwinding—of the resolver. In this case the stator voltages are composedof a carrier frequency part and rotational angle dependent amplificationfactor (cf. the respective signal diagrams in the FIGURE).

The control loop for the stator windings is configured as follows.

The rotational angle dependent amplification factor is determined by acontroller 2 having an integrator 3 at the downstream side. Thecontroller 2 receives as an input variable the difference between thetarget value and the demodulated voltage U_(err) of the rotatingexcitation winding 13 a of the resolver. The controller 2 provides as anoutput variable a rotation speed that is integrated in the downstreamintegrator 3 to obtain an angle ε′ (or ε*). If the control loop isadjusted (operated functionally) then the obtained angle ε′ correspondsto the real angle ε and the demodulated voltage U_(err) is zero Volt (asalternating voltage).

The excitation winding 13 a may be a group of windings 13 a, 13 b and 13c. Its measurement signal is for the input amplifier 11.

The following equations describe these connections

Rotation Angle of the Rotor:εObtained Rotation Angle:ε′Carrier Frequency Signal:U ₁ =û·sin(ω·t)Signals of the Stator Windings:U ₁ =û·sin(ω·t)·cos ε′U ₂ =û·sin(ω·t)·sin ε′Signal at the Excitation Winding:U _(err)=(U ₁·sin ε+U ₂·cos ε)·üTransformation Ratio of Windings:ü

In the adjusted state ε=ε′. Thus, this yieldsU _(err)=(û·sin(ω·t)·cos ε′·sin ε+sin(ω·t)·sin ε′·cos ε)·üU _(err) =û·sin(ω·t)·(cos ε′·sin ε+sin ε′·cos ε)·üU _(err) =û·sin(ω·t)·(sin(ε′−ε))·üU _(err)=0,wenn ε′≈ε

If, for example, ü=0 due to a failure in the rotor of the resolver R,then U_(err) is also equal to zero and the control loop is withouteffect, this, however, not being recognizable.

In order to solve this problem the signals U₁ and U₂ are to be set tozero in an alternating manner and to force a transmission to the“excitation side” (as measurement winding 13). This happens inone-second intervals by means of pulse signals δ_(i) of the interferencesignal generator 15.

If one sets one signal, either U₁ or U₂, equal to zero, then thefollowing U_(err) is obtainedU ₁=0:U _(err) =û·sin(ω·t)·(sin ε′·cos ε)·üU ₂=0:U _(err) =û·sin(ω·t)·(cos ε′·sin ε)·ü

If ü≠0 (the resolver is electrically in a correct state), then during aninterference signal δ_(i) then the measured U_(err) is also ≠0→ it canbe analysed by block 16. This block is coupled to the interferencesignal generator 15 for the time coordination.

Since also the angle ε may cause the factor cos ε or sin ε to be equalto zero, this uncertainty is circumvented by an alternating nulling ofU₁ and U₂.

Since the switching off (as an interference signal) of the carrierfrequency of the carrier frequency generator 8 for one of the twomanipulated variables U₁, U₂ interferes with the demodulation of thedemodulator 9 and thus disturbs the controller 2, the control activityis thus “stopped” during this time (the switching off of the carrierfrequency). This is accomplished via the switch 17. It disrupts the feedback and “temporarily” ends the controlling. To this end a toggle switchcan inject into the difference location 11 a zero signal as ameasurement signal.

In addition, the time period of switching off may be adapted to thefilter characteristics of the electric system in order to reduce theduration of transient oscillations.

The signal names u₁₁, u₁₂, u₁₃ and others are evident from FIG. 1 andthe position shown therein, where they occur.

The invention claimed is:
 1. A method of monitoring a system having aresolver in a control loop, the resolver coupled to an electric machine,the method comprising the steps of: transmitting two angle dependentamplitude-modulated carrier frequency signals to two stator windings ofthe resolver; analyzing a carrier frequency signal response via an inputamplifier providing an output signal at an output thereof, the outputsignal being substantially equal to zero if the amplitude-modulatedsignals are both functionally generated; generating a time intervalinterference signal alternating for the amplitude-modulatedcarrier-frequency signals fed to the two stator windings of theresolver; wherein as a result of the interference signal, aninterference indication signal is present at the output of the inputamplifier as a signal that is different from zero if an error or a wirebreakage does not exist, and as a signal that is equal to zero if anerror or a broken wire exists within the system.
 2. The method of claim1, further comprising: providing a failure detection circuit for asafety related evaluation and monitoring of the resolver, the failuredetection circuit having a generator generating the interference signalin or for the two stator windings of the resolver.
 3. The method ofclaim 2, wherein the failure detection circuit comprises a signalanalysis section analyzing the output signal of the input amplifier andprovides a signal for failure detection while the control loop istemporarily disabled by a switch device.
 4. The method of claim 3,wherein a switch of the switch device blocks or switches off a feedbackof the output signal of the input amplifier in a feedback branch of thecontrol loop.
 5. The method of claim 4, wherein the switch is a togglingswitch.
 6. The method of claim 1, wherein the amplitude-modulatedsignals provide a voltage at each of the stator windings of theresolver, an amplitude of each the voltage being continuously adjustedduring normal operation such that an induced voltage at one rotorwinding as excitation winding of the resolver is zero.
 7. The method ofclaim 1, wherein the resolver is operated in a backward mode and iscoupled to the electric machine, said control loop includes a controldifference unit, a controller connected to an amplitude modulator via anintegrator, wherein said amplitude modulator comprises a sine signalcircuit, a cosine signal circuit, and two multipliers and a carrierfrequency generator, and two amplifiers supply currents to the twostator windings of the resolver.
 8. The method of claim 1, wherein asignal induced in an excitation winding as a receiver winding of theresolver is supplied, via the input amplifier and a demodulator, asfeedback to the control loop.
 9. The method of claim 1, furthercomprising providing a failure detection circuit that couples theinterference signals into a carrier frequency generator during giventime periods in one-second intervals, the interference signalsoriginating from an interference signal generator, thereby disturbing ornulling a respective one of the carrier frequency signals provided bythe carrier frequency, generator.
 10. The method of claim 9, thedisturbing provides including of the interference signal into the inputsignal of the input amplifier or generating an input signal as analternating signal at the input amplifier.
 11. The method of claim 1,wherein, for functional generation of the amplitude-modulated signals,the output signal of the input amplifier is an alternating signal thatis substantially equal to zero.
 12. The method of claim 1, wherein theoutput signal of the input amplifier is an alternating signal during alength of time of the interference signal.
 13. A method of monitoring arotational angle system having a rotational angle sensor operativelycoupled to an electric machine, comprising the steps of transmitting twoamplitude-modulated carrier frequency signals to two stationary windingsof the rotational angle sensor; providing an input amplifier supplyingan output signal at an output thereof, the output signal beingsubstantially equal to zero upon functionally properly generating bothcarrier frequency signals; generating an interference signal alternatingfor both carrier frequency signals fed to the stationary windings of therotational angle sensor, thereby creating an interference indicationsignal at the output of the input amplifier, as a signal that is notequal to zero when no error or no breakage is present within therotational angle system, and as a signal that is equal to zero when anerror or a breakage is present within the rotational angle system. 14.The method of claim 13, further comprising providing a failure detectioncircuit for a safety related monitoring of the rotational angle sensor.15. The method of claim 14, said failure detection circuit having acombination of an interference signal generator, a signal analysiscircuit, and a switch device, the failure detection circuit beingconnected to a carrier frequency generator supplying the carrierfrequency signals.
 16. The method of claim 14, further comprisingperforming a further analysis and monitoring of the rotational anglesensor after a failure detection of the failure detection circuit. 17.The method of claim 13, wherein voltage amplitudes at each of the statorwindings of the rotational angle sensor are adjusted by a control loopsuch that a measured voltage is zero at an excitation winding used as areceiver winding of the rotational angle sensor.
 18. The method of claim13, wherein the stationary windings are oriented orthogonally to eachother.
 19. The method of claim 13, wherein the amplitude-modulatedsignals as carrier frequency signals are angle-dependent.
 20. A circuitassembly for monitoring a rotational angle sensor at an electricmachine, said circuit assembly having a control difference unit, acontroller connected via an integrator to an amplitude modulator havinga sine signal function and a cosine signal function, two multipliers anda carrier frequency generator, wherein said frequency generator isconnectable, via two amplifiers, with stator windings of the rotationalangle sensor, an excitation winding thereof being connectable to ademodulator via an input amplifier, and comprising a failure detectioncircuit having a combination of an interference signal generator, asignal analysis circuit and a switch device, wherein the failuredetection circuit is connected to the carrier frequency generator. 21.The circuit assembly of claim 20, wherein the rotational angle sensor isconfigured as resolver.
 22. The circuit assembly of claim 20, whereinthe interference signal generator is configured to provide interferencesignals to the carrier frequency generator in one-second intervals. 23.The circuit assembly of claim 20, wherein the generation of theinterference signals is time-coupled to the switch device for disablinga control loop, the control loop comprising the control difference unit,the controller, the integrator and an amplitude modulator, themultipliers, and the two amplifiers, the stator windings, the excitationwinding, the demodulator and input amplifier and the switch device. 24.A device for monitoring a resolver operated in a backward mode at anelectric machine, comprising: a control difference unit; a controllerconnected, via an integrator, to an amplitude modulator, the amplitudemodulator having a sine signal unit and a cosine signal unit and twomultipliers; a carrier frequency generator; two amplifiers that supplycurrent to stator windings of the resolver; wherein a signal induced ina rotatable winding provides a feedback signal of a control loop via aninput amplifier, a demodulator, and a switch; a failure detectioncircuit including an interference signal generator configured to couplean interference signal into the carrier frequency generator duringdefined time periods; wherein an output signal of the input amplifier isevaluated by a signal analysis circuit for a failure detection, while oras long as the control loop is temporarily disabled by the switch.